Coloured shadows

You will need

• 3 small torches
• Red, blue and green cellophane
• Baking paper
• Scissors
• Sticky tape
• White, vertical surface (such as a wall or door)
• Small object (such as a cork or a salt shaker)

What to do

1. Roughly measure the size of the front end of each one of your torches.
2. Cut a square of red cellophane large enough so it will completely cover the end of the torch. Repeat this step by estimating and cutting a similar square of baking paper.
3. Place the cellophane over the end of the torch and sticky tape its edges in place, so the light can pass through it. Place the baking paper over the cellophane and sticky tape it in place as well.
4. Repeat the above steps for the blue and green cellophane, giving you three torches that shine red, green and blue light. The baking paper diffuses the light, giving you an even glow.
5. Find a dark room and a white surface. Turn on each torch and shine them on the same spot. What colour is the light?
6. Put the small object in front of the torches. How many shadows can you see? What colour are they?
7. Shine only two torches on the same spot. What happens to the colours?

What's happening?

Imagine the light coming out of the torch as waves rippling across a pond. Some of these are tiny ripples. Others are long bumps. These are described as 'wavelengths' of light.

To see these wavelengths as light, we use a type of cell inside your eyeball called a 'cone'. When it is hit by a wave of light, it sends a message to the brain which helps it to create an image.

Cones come in three varieties; one responds to the light waves that are shortest (called an 'S' cone); another to light waves that are longest (called an 'L' cone); and a third to waves that are sized somewhere in between (or the 'M' cone).

When an S cone cell sends a lot of messages to the brain, we recognise it as the colour blue. When L cone cells do this, we see red. The third type, M, creates what we see as the colour green.

To see other colours, the cells must send a combination of messages. For example, if L and M both send messages at the same time, the brain sees the colour yellow. This can happen when there is a mix of long and medium sized waves, such as when you shine both the red and green torch on the same patch of wall. If S and M cells send messages together, such as when blue and green light mixes, the brain might see a colour called 'cyan'. L and S cone cells acting together can produce a pinky-purple colour called magenta.

Shining all three colours on the same patch makes a mix of short, medium and long waves, which affects all three types of cones. When this happens, the brain interprets the mixture of messages from the eye as white.

Applications

When painting a picture, you can make green by mixing yellow and blue paints. This is called 'colour by subtraction' – the particles in the blue and yellow paint absorb (or 'subtract') different colours from the surrounding light until only green is left, which bounces out of the paint and into your eye.

In subtractive mixtures, there are three primary colours; red, blue and yellow. From these, nearly all hues of colour can be made. Adding white or black paint can produce different shades.

Combining coloured lights, however, is different. In this case, yellow isn't a primary colour. Instead, it is produced by mixing red and green light. This is called 'colour by addition'; light is added together to produce colour, rather than subtracted from the white light shining on a painted surface.

Combining three primary colours of paint in equal amounts usually results in a brown-grey coloured mess. On the other hand, mixing the primary colours of light (red, blue and green) makes a beautiful spot of pure white light.